The present invention relates to filters and, more particularly, to filters suited to be realized in integrated circuit form which utilize differential transconductance amplifiers.
Prior art filters utilizing transconductance amplifiers of the type disclosed in U.S. Pat. No. 3,692,650 in which the gain is variable are known. These filters utilize transconductance amplifiers in a single ended configuration and generally include an integrating capacitor coupled across the output to thereby form a filter cell. By cascading filter cells a multiple filter can be derived which, for example, may be used to provide video signal filtering. For instance, a two-pole filter comprising a pair of single ended transconductance amplifiers connected in series is shown in Vol. CE-32, #3, August '86, IEEE Transaction On Consumer Electronics. Feedback is provided from the output of the two-pole filter to the individual filter cells.
It is desirable to be able to control the damping factor of these type of filters in order to control the pass band and out of band cut-off of the filter among other characteristics thereof. This desirability is limited by the prior filters using single-ended configured transconductance amplifiers. In these types of filters, in order to change the damping factor, either the ratios of the gm of the amplifiers or the capacitor ratios of the integrating capacitors must be varied since the damping factor is a function thereof. In most prior art such filters the gm of each of the transconductance amplifiers are made equal to one another which means that to control the damping factor the ratios of the integrating capacitors must be varied. The problem faced in the prior art is that to obtain a damping factor significantly different from a value of one-half requires a large change in the ratio of the capacitors which may not be achievable if these types of filters are to be fabricated in integrated circuit form. Alternately, ratioing gm values tends to cause poor control over the ratio as gms are changed. In addition, typical prior art filters of the foregoing type in general have the integrating capacitors coupled between the output of the transconductance amplifiers and either ground or the power supply rail. This creates an additional problem in that the loop stability is poor.
Hence, a need arises to be able to realize in integrated circuit form at least a two-pole filter which utilizes cascaded transconductance amplifier stages in which the damping factor can be controlled without requiring large changes in capacitor ratios and in which the integrating capacitor or portion thereof can be used to compensate a common mode loop that sets the common mode output of the filter.